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Article

Unusual Catalytic Effect of Fe3+ on 2,4-dichlorophenoxyacetic Acid Degradation by Radio Frequency Discharge in Aqueous Solution

College of Environmental Science & Engineering, Dalian Maritime University, Dalian 116026, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(11), 1719; https://doi.org/10.3390/w14111719
Submission received: 2 May 2022 / Revised: 20 May 2022 / Accepted: 23 May 2022 / Published: 27 May 2022
(This article belongs to the Topic Advanced Oxidation Process: Applications and Prospects)

Abstract

:
2,4-dichlorophenoxyacetic acid (2,4-D) is a widely used herbicide for controlling broad-leaved weeds. The development of an efficient process for treating the refractory 2,4-D wastewater is necessary. In this study, liquid-phase degradation of 2,4-D induced by radio frequency discharge (RFD) was studied. Experimental results showed that the degradation was more effective in acidic than in neutral or alkaline solutions. During the degradation, a large amount of hydrogen peroxide (H2O2, 1.2 mM/min, almost equal to that without 2,4-D) was simultaneously produced, and catalytic effects of both ferric (Fe3+) and ferrous (Fe2+) ions on the degradation were examined and compared. It was found that 2,4-D degraded more rapidly in the case of Fe3+ than the that of Fe2+. Such a scenario is explained that Fe3+ was successively reduced to Fe2+ by the atomic hydrogen (•H) and •OH-adducts of 2,4-D resulting from RFD, which in turn catalyzed the H2O2 to form more •OH radicals through Fenton’s reaction, indicating that Fe3+ not only accelerates the degradation rate but also increases the amount of •OH available for 2,4-D degradation by suppressing the back reaction between the •H and •OH. 2,4-dichlorophenol, 4,6-dichlororesorcinol, 2-hydroxy-4-chloro- and 2-chloro-4-hydroxy- phenoxyacetic acids, hydroxylated 2,4-Ds, and carboxylic acids (glycolic, formic and oxalic) were identified as the byproducts. Energy yields of RFD have been compared with those of other nonthermal plasma processes.

1. Introduction

2,4-dichlorophenoxyacetic acid (2,4-D) is a systemic herbicide extensively used for broad-leaved weeds control in cereal crops, pastures, and orchards. 2,4-D kills dicots (but not grasses) by mimicking the growth hormone auxin, causing uncontrolled growth and eventual death of the susceptible plants [1]. In 2015, the International Agency for Research on Cancer (IARC) confirmed its 1987 classification of 2,4-D as a Group 2B carcinogen [2,3]. Due to its massive use and high solubility in water, 2,4-D can easily seep into the aquatic environment during its production and application. 2,4-D is hardly biodegradable at concentrations higher than 1.0 mg/L [4]. Acid-washed powdered activated carbon (PAC) can effectively adsorb 2,4-D from aqueous media [5]. However, as PAC adsorption is just a phase transferring process, the adsorbent becomes a hazardous waste when it is saturated with pollutants that need to be properly disposed of. In this context, some energetic methods such as TiO2 photocatalysis [6], electrochemical oxidation [7], gamma irradiation [8], sonolysis or their combination [9] have been developed. Although showing some promising, these processes suffer respective deficiencies of either process complexity or low energy utilization, and the search for efficient and effective 2,4-D decomposition processes is necessary.
In the past decades, electrical discharges for water purification have been investigated extensively [10,11,12]. Pulsed corona discharge (PCD) inside water, glow discharge in contact with water and dielectric barrier discharge (DBD) close to water surface etc. were usually employed for the decomposition of harmful pollutants [12]. Degradation of 2,4-D by PCD over water surface [13,14] and by DBD in planar falling film reactor [15] have been evaluated and compared with some typical water treatment technologies. Recently, radio frequency discharge (RFD) [16,17] is receiving increasing attention as a novel plasma water treatment process due to its easy formation both in pure water and in solution with high conductivity [17], low breakdown voltage [18] and experimental simplicity. Several chemically active radicals (•OH, H•, •O, etc.) derived from H2O molecules were in situ generated, which can initiate the degradation of organic pollutants such as methylene blue [16] and Congo Red [18]. Wang et al. investigated the RFD of metronidazole [19] and found that reducing species played a significant role during the degradation. The formation of hydrogen peroxide (H2O2) during RFD of pure water was reported, and some catalytic effects of ferrous sulfate on the dye discoloration were observed. However, H2O2 quantification with the presence of the dye was not performed. In addition, when ferrous ion (Fe2+) was added to the solution, it would be rapidly oxidized by H2O2 into ferric ion (Fe3+) [16]. However, at present, no comparison in catalytic effect between Fe2+ and Fe3+ has been made. In this study, both 2,4-D degradation and H2O2 formation were investigated. Besides optimizing the experimental conditions, the catalytic mechanism of iron salts, especially that of the Fe3+ on the degradation was fully explored.

2. Experimental

The experimental apparatus, consisting of a radio frequency power supply (RSG-300, Reshige Electronics Inc., Changzhou, China), a vacuum matching box, a quartz reactor and a coaxial electrode system, is shown in Figure 1.
The reactor was a quartz cylinder with 60 mm of inner diameter and 150 mm in height. The coaxial electrode system consists of an inner electrode (pointed Pt wire, Φ = 1.0 mm), a quartz tube (i. d. = 1.0 mm, o. d. = 2.0 mm), and an outer electrode (copper tube, i. d. = 2.2 mm, o. d. = 3.0 mm). Discharge is generated at the tip of the inner electrode in contact with the liquid. In such an electrode system, the discharge can be stably sustained without the shielding box needed in conventional RFD [18,19]. 2,4-D was dissolved in distilled water and a 250 mL portion was poured into the reactor for treatment. Na2SO4 was used to adjust the solution conductivity. The pH of the solution was adjusted with dilute H2SO4 or NaOH to the expected value. During the reaction, the plasma heats the water in the reaction cell [16]. The reaction vessel was coated by a water jacket, where the temperature of the solution was kept at 298 ± 2K by running with cooling water.
During the discharge, the solution in the reactor was constantly agitated by a magnetic stirrer and the aliquots were periodically taken out for analysis. 2,4-D and the phenolic byproducts were analyzed by a reversed-phase HPLC (SHIMADZU LC-20A) coupled with a diode array UV detector. A C18 (4.6 × 25 mm) column was used for the separation. The eluent was composed of 40% acetonitrile and 59.9% H2O and 0.1% H3PO4. The flow rate was 1.0 mL/min. The detection wavelength was usually set at 283 nm. Removal of 2,4-D ( η 2 , 4 D ) was calculated according to Equation (1).
η 2 , 4 - D ( % ) = ( C 0     C t ) C 0 × 100
where Ct (mM) was the concentration of 2,4-D at discharge time t and C0 (mM) was the initial concentration of 2,4-D. Energy yields for 2,4-D removal (J2,4-D) is calculated as Equation (2) [11,14].
J 2 , 4 - D = C 0 V o l 2 P t 1 / 2
where Vol represents the solution volume (L), P the input power (W) and t1/2, the discharge time required for 50% removal of 2,4-D (s).
The phenolic byproducts were identified by comparison of their retention times and UV spectra with those of the authentic standard samples or with the help of LC-MS [20]. Determination of organic acids and chloride ions (Cl) was performed by ion chromatography (IC, DIONEX ICS-1100) equipped with an Ion Pac AG-23 column. 0.01 mol/L KOH solution was used as the eluent with a flow rate of 1.0 mL/min. H2O2 was determined by a colorimetric method where the yellow color pertitanic acid formed by mixing the treated solution with titanium sulfate was determined by absorption at 410 nm [21]. The concentration of Fe2+ was determined colorimetrically where 1,10-phenanthroline was used as the color reagent [22]. It is noted that some catalase should be immediately added to the solution to decompose the residue H2O2 as it interferes with the determination of the Fe2+. Total organic carbon (TOC) was measured by a TOC analyzer. The gaseous hydrogen evolved from the solution was determined by gas chromatography (GC-2014C, SHIMADZU). Each determination was performed three times and the average value was adopted, with a standard deviation of less than 4.5%.

3. Results and discussion

3.1. 2,4-D Degradation and Chloride Formation

2,4-D undergoes efficient degradation when subjected to RFD. Figure 2 shows the decays of 2,4-D and TOC and the formation of Cl during the RFD treatment.
As depicted in Figure 2, both 2,4-D and TOC declined exponentially while the yield of Cl increased gradually with increasing discharge time. The TOC decreased less rapidly than 2,4-D, indicating that intermediate byproducts were formed during the discharge. At the initial stage (<60 min), the yield of Cl is less than that expected from the decomposition of the parent compound. With 60 min of discharge treatment, 61% of 2,4-D was degraded and 0.32 mM of Cl was generated, meaning that only 26% of the organic chlorine of the transformed 2,4-D was mineralized into Cl. After 140 min of discharge, the concentration of 2,4-D was below the detection limit and 1.63 mM of Cl was generated, where the yield of Cl reached 82%. After 210 min, the Cl reached its stoichiometric point (2.0 mM), and after 300 min. (not shown in the figure), the concentration of TOC reached zero, indicating that 2,4-D was totally transformed into inorganic carbon and Cl under the action of RFD.

3.2. Effects of Initial pH on 2,4-D Degradation

pH is an important parameter affecting the degradation process. In order to better elucidate the effects of pH on 2,4-D degradation, pH variations during the RFD in the presence and absence of 2,4-D are illustrated in Figure 3.
As shown in Figure 3, without 2,4-D, the solution pH varied little during the discharge. However, in the presence of 2,4-D, the pH dropped dramatically. The decrease of pH in presence of 2,4-D can be explained by the formation of HCl and organic acids (c.f. 3.4). In order to better examine the effect and mechanism of pH on 2,4-D degradation, the experiments were performed both in buffered and unbuffered solutions. Here, phosphates were chosen as the buffering agents as the rate constants for the reactions of phosphates with •OH (e.g., Equations (3) and (4)) [23] are much smaller than that for the reaction of SO42− (Equation (5)) [14].
H2PO4 + •OH → H2PO4• + OH   k3 = 2.0×104 M−1·s−1
HPO42−+ •OH → HPO4• + OH   k4 = 1.5×105 M−1·s1
SO42− + •OH → SO4• + OH      k5 = 1.5×106 M−1·s−1
Figure 4 shows the 2,4-D removal under different initial pH (pH0) values with 10 min of RFD treatment in unbuffered and phosphate buffered (prepared with 2.0 mM NaH2PO4, no data from pH0 3.0 to 6.0 as it has little buffering in the range) solutions.
It can be observed from Figure 4 that the 2,4-D removal decreases with increasing pH0, both in the cases of the buffered and the unbuffered solutions. The removal of 2.4-D declined from 37% at pH 2.0 to almost half (18%) at pH0 9.0 and from 33% at pH 2.0 to 13% at pH0 8.0, meaning that the decrease is more pronounced in the buffered solution. Such phenomena can be explained that the degradation of 2.4-D in RFD is mainly an •OH radical process (degradation rate decreased in the presence of ·OH scavenger, not shown in the figure), [20] it is more advantageous to the decomposition under acidic conditions as the alkoxyl group is more easily to be released from the C1 ·OH adducts in the presence of H+ [24,25] (Scheme 1):
Figure 4 shows that the difference in 2,4-D degradation between pH 4.0 and 9.0 was less noticeable in the case of unbuffered solution than in buffered ones, possibly due to the formation of acids which reduced the pH gap in the unbuffered system.

3.3. H2O2 Formation and Effects of Iron Salts on 2,4-D Degradation

Previous studies showed that the major molecular product formed in the liquid phase is H2O2. [16,17] However, the formation of H2O2 in the presence of organic pollutants has rarely been reported. [16] In this investigation, H2O2 generated both in the presence and absence of 2,4-D is presented in Figure 5.
It can be seen in Figure 5 that less H2O2 was generated in 2,4-D solution than in solution without it, meaning that •OH radicals are the primary precursors of H2O2 and are the active species for 2,4-D degradation. However, the discrepancy is little in the early stage (<15 min), and the gap between the two concentrations of H2O2 was less than 10%, meaning that the concentration of •OH radicals near the plasma is so high that only a small part of them was consumed by 2,4-D and the rest dimerized to form H2O2.
The concentration of H2O2 in the presence of 2,4-D traces the curve for experiments in its absence, indicating that H2O2 alone cannot lead to rapid degradation of 2,4-D. It is desirable to convert H2O2 into reactive •OH radicals for pollutant degradation. It is desirable to add iron ions to enhance the •OH radical formation and increase the 2,4-D decomposition through Fenton’s reaction (Reactions (6) and (7)) [26].
Fe2+ + H2O2  →  Fe3+ + •OH + OH   k6 = 40–80 M−1·s−1
Fe3+ + H2O2  →  Fe2+ + HO2• + H+ k7 = 0.001–0.01 M−1·s−1
The 2,4-D degradation in the presence of 0.5 mM iron ions is shown in Figure 6.
It can be observed from Figure 6 that both Fe2+ and Fe3+ showed remarkable catalytic effects on 2,4-D degradation. With 5 min of discharge, 2,4-D removal in the presence of Fe3+ (38%) was a little higher than that of Fe2+ (36%), where it was only 20% without a catalyzer. However, the catalytic effect of Fe3+ was more obvious than that of Fe2+ with increasing discharge time. At 10 min. the 2,4-D was totally removed in Fe3+ while only 70% was achieved for Fe2+. Such a phenomenon is abnormal in conventional Fenton reactions, as the reaction (7) is several orders of magnitude slower than the reaction (6). The reason may be that Fe3+ has been reduced by the ·OH-adducts formed by addition to the ring at positions unoccupied by the substituted groups (hydroxycyclohexadienyl radicals), producing the corresponding hydroxylated 2,4-Ds and Fe2+ [25,27] (Scheme 2):
On the other hand, in the present pH range, much of Fe3+ is present in the form of FeOH2+, which can be efficiently reduced by the ·H atoms generated by the RFD to Fe2+ [28]:
·H + FeOH2+ → Fe2+ + H2O
Fe2+ ion resulting from the reactions above reacts with H2O2 (reaction (6)) to convert the H2O2 back to ·OH radicals, ultimately leading to the decrease in H2O2 formation (Figure 5) and the increase in 2,4-D degradation (Figure 6) [13].
In order to prove the above assumptions, formations of Fe2+ and molecular hydrogen (H2) during RFD were investigated, and the results were shown in Figure 7a and b, respectively.
It can be observed from Figure 7a that, in the case of Fe3+, a lot of Fe2+ was formed during the RFD in the presence of 2,4-D and little Fe2+ was detected in the absence of 2,4-D, indicating the UV radiation from RFD is too weak to effectively induce the photoFenton activation (FeOH2+ + UV → Fe2+ +·OH) [29]. Figure 7b shows that the amount of H2 in the gas phase decreased in the presence of Fe3+. At 10 min. about 0.3 mmol H2 was evolved from the solution in the absence of Fe3+. However, only 0.19 mmol H2 was detected when 0.5 mM Fe3+ was added to the solution, which means that at least 0.22 mmol Fe3+ was reduced to Fe2+ by the ·H atoms. As the reaction between ·H and ·OH is suppressed due to the scavenging effect of Fe3+ [30], which in turn increases the amount of ·OH radicals available for 2,4-D degradation.
H +·OH → H2O
However, as shown in Figure 7b, the amount of H2 formed with the case of Fe2+ is higher than that without it. This can be explained that the •OH produced by RFD can rapidly oxidize the Fe2+ to Fe3+, which suppressed the Reaction (9) and increase the amount of ·H and ultimately the amount of H2 via dimerization [30].
OH + Fe2+ → Fe3+ + OH
H + H → H2
Fenton reagent (Fe2+ + H2O2) oxidation is an effective method for degradation of the refractory organic pollutants. However, the reaction has to be undertaken in acidic conditions (pH: 2–4). In addition, as the Fenton reaction proceeds, the Fe2+ rapidly oxidizew to Fe3+ and the reaction becomes a Fenton-like one, which leads to a substantial decrease in the reaction rate. In order to accelerate the reaction, ultraviolet light (UV) was usually employed (UV/Fenton). However, UV/Fenton was only effective in solutions with high UV transmittance. In RFD of 2,4-D, Fe3+ showed an extraordinary catalytic effect due to its successive reduction to the Fe2+, meaning that RFD may be used to promote the Fenton-like reactions, especially in liquids with poor UV transmittance, which is of interest because most real wastewater is UV opaque. In addition, the pH of the solution can spontaneously drop to the Fenton’s operation range, indicating that Fe3+/RFD is a promising process for 2,4-D degradation. In order to further clarify the potential application of RFD in accelerating the Fenton-like process, concentrated 2,4-D degradation by Fe3+/H2O2 in the presence and absence of RFD was performed, and the results were shown in Figure 8.
It can be observed from Figure 8 that without RFD, little 2,4-D degradation proceeded. When RFD was applied in the Fenton-like process, the 2,4-D degradation was significantly enhanced where it could be removed completely in 25 min. of RFD, indicating that RFD was a useful tool for accelerating the Fenton-like process.

3.4. Intermediate Products Formation and Possible Decomposition Pathway

In order to gain insight into the 2,4-D decomposition mechanism, HPLC and IC were utilized to help identify the intermediate products. Figure 9 shows the HPLC chromatograms of the solution within 40 min of RFD treatment monitored at 283 nm.
As shown in Figure 9, several byproducts were observed. The peaks at retention times 12.8 min and 23.5 min are 4,6-dichlororesorcinol (4,6-DCR) and 2,4-Dichlorophenol (2,4-DCP), respectively. Three isomers of hydroxylation products of 2,4-D, i.e., 3-, 5- and 6-hydroxy-2,4-D (C8H6Cl2O4) and two chlorohydroxyl isomers (2-hydroxy-4-chloro- and 2-chloro-4-hydroxyphenoxyacetic acid: C8H7ClO4) were roughly identified by the LC-MS [20]. As no standards are available for these products, their precise identification and quantification cannot be performed. As 2,4-DCP is found as a common intermediate byproduct in other methods, [6,7,8,9] the concentration variation of 2,4-DCP as a function of discharge time is given in Figure 10.
It can be observed from Figure 10 that the 2,4-DCP forms immediately after the discharge takes place. Its concentration increases gradually with discharge time trough to 90 min and then progressively diminishes. Such a gradual increasing and then decreasing trend is always observed in other treatment processes. The maximum concentration of 2,4-DCP formed is 0.11 mM in RFD of 1.0 mM 2,4-D. In comparison, more than 0.5 mM 2,4-DCP was formed in TiO2 photocatalysis of 1.0 mM 2,4-D [6], 0.04 mM in γ radiolysis of 0.5 mM 2,4-D [8] and 0.02 mM in sonolysis of 0.22 mM of 2,4-D [9], respectively, indicating that the 2,4-DCP buildup/decomposition pattern in RFD is similar to those in γ radiolysis and sonolysis (the maximum concentration of 2,4-DCP is about 10% of the parent compound).
Aliphatic acids with a retention time of less than 3 min are difficult to quantify as HPLC cannot separate them well. Therefore, IC was employed to determine the organic acids, and results are shown in Figure 11 and Figure 12, respectively.
As shown in Figure 11, glycolic acid, formic acid and oxalic acid were produced. The yields of formic acids and glycolic acid increased to the maximums within 120 min and then diminished gradually. On the other hand, the yield of oxalic acid increased smoothly within 180 min and then declined. At 120 min, they are present with: glycolic acid at 0.31 mM, formic acid at 0.20 mM, and oxalic acid at 0.13 mM (Figure 12).
Figure 12 shows that the amount of glycolic acid accounts for almost half of the degraded 2,4-D, indicating that its formation results from the primary reaction step. Glycolic acid was formed by cleavage of ipso-•OH-adducts on position 1 of 2,4-D [25] (Scheme 3):
Formic and oxalic acids are common ring opening byproducts of aromatic compounds in advanced oxidation processes [12]. However, it should be noted that acetic acid observed in the ionizing radiation of 2,4-D [20] was not detected in the present investigation. This may be because the retention time of acetic acid is very close to that of glycolic acid in IC.
The formation of chloride ion (Cl) was attributed to the addition of •OH on the position of 2 or 4, followed by the fast elimination of HCl [20] (Scheme 4):
•OH radicals addition to the unsubstituted position of 2,4-D and then oxidized by phenoxyl radical to produce the hydroxylated 2,4-Ds and 2,4-dichlorophenol or chlorohydroxyphenoxyacetic acid [31] (Scheme 5):
Further hydroxylating of 2,4-DCP or ydroxyl-2,4-D leads to the formation of 4,6-DCR [32] (Scheme 6):
It is noted that typical phenolic byproducts such as 4-chlorocatechol, 2-chlorohydroquinone and 1, 2, 4-trihydroxybenzene commonly found in other radical processes were not observed in the present investigation, possibly because of their unstable nature [7].
Based on the above observations, the probable reaction mechanism for 2,4-D decomposition by RFD is proposed in Scheme 7.
Scheme 7 shows that degradation of 2,4-D in RFD proceeds as follows: initiation of the reaction, ring destruction and oxidation of the organic acids. At the beginning stage, •OH radicals resulting from the RFD reacting with 2,4-D, forming the hydroxycyclohexadienyl radicals (•OH adducts). The •OH adducts formed on ipso positions subsequently undergo HCl and glycolic elimination, forming the corresponding phenoxyl radicals. The resulting phenoxyl radicals are unstable and rapidly oxidize the •OH adducts of the unsubstituted positions to form the corresponding phenols (2,4-DCP, 2-hydroxy-4-chloro- and 2-chloro-4-hydroxy- phenoxyacetic acids) and hydroxylated 2,4-Ds. Further hydroxylating of the aromatic ring leads to the ring cleavage to form the organic acids. The organic acids are eventually oxidized by •OH radicals into CO2 and H2O. As indicated in Figure 6, the solution would be acidic at the end of the discharge if the 2,4-D decomposition was conducted in neutral pH. Therefore, the final product in the solution would be HCl.

3.5. Energy Yields Evaluation

Energy yields (J2,4-D) is a significant factor in evaluating the feasibility of practical application. J2,4-D is calculated according to Equation (2). J2,4-D and reaction rate constant (k, min−1) [11] of RFD, other nonthermal plasma systems and some competitive methods are summarized in Table 1.
Table 1 shows that the J2,4-D of RFD is comparable to that of sonolysis and is higher than that of TiO2 photocatalysis. Among the nonthermal plasma systems, J2,4-D follows the order of DBD/Ar-Fenton ≈ RFD-Fenton like > DBD/Ar > PCD/Air ≈ RFD/Fe3+ > DBD/Air. It is shown that the J2,4-D of RFD/H2O2/Fe3+ is comparable to that of DBD/Ar-Fenton, the most efficient nonthermal plasma process reported. However, DBD/Ar-Fenton has to be operated in a pure noble argon atmosphere, instead of air, which would greatly increase the experimental complexity and the operation cost, indicating that RFD is not only a competitive process for 2,4-D degradation itself but also a useful tool to accelerate the Fenton-like process.

4. Conclusions

2,4-D can be efficiently degraded and dechlorinated under the action of RFD. The successive attack of •OH radicals on the benzene ring was the key step. Decreasing solution pH was favorable for the degradation. During the discharge, pH gradually decreases as HCl and organic acids are formed, and significant quantities of hydrogen peroxide are produced. Both Fe3+ and Fe2+ displayed marked catalytic effects. However, due to the reducing actions of the •OH-adducts and the hydrogen atoms, Fe3+ performed better than Fe2+, which indicates that RFD may also be used to promote the Fenton-like reactions. Major intermediate byproducts are 2,4-dichlorophenol, 4,6-dichlororesorcinol, hydroxylated 2,4-Ds, 2-hydroxy-4-chloro- and 2-chloro-4-hydroxyphenoxyacetic acids, glycolic acid, formic acid, oxalic acid and chloride ion. RFD has comparable energy yields with those of pulsed corona discharge over a water surface and sonolysis and is higher than that of TiO2 photocatalysis, indicating that RFD is a useful process for 2,4-D decomposition.

Author Contributions

Conceptualization, Y.L.; methodology, Y.L.; software, Y.L.; validation, Y.L.; formal analysis, Y.L.; investigation, Y.L.; resources, B.S.; data curation, Y.L.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L.; visualization, Y.L.; supervision, Y.L.; project administration, Y.L.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China grant number [11005014, 11675031] and The APC was funded by [11005014].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful for the support of the National Natural Science Foundation of China (11005014, 11675031).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Available online: https://www.invasive.org/gist/products/handbook/10.24-d.pdf (accessed on 24 April 2001).
  2. Loomis, D.; Guyton, K.; Grosse, Y.; Ghissasi, F.E.; Bouvard, V.; Benbrahim-Tallaa, L.; Guha, N.; Mattock, H.; Straif, K. Carcinogenicity of lindane, DDT, and 2,4-Dichlorophenoxyacetic acid. Lancet Oncol. 2015, 16, 891–892. [Google Scholar] [CrossRef]
  3. Song, Y. Insight into the mode of action of 2,4-Dichlorophenoxyacetic acid (2,4-D) as an herbicide. J. Integr. Plant Biol. 2014, 56, 106–113. [Google Scholar] [CrossRef] [PubMed]
  4. Hoover, D.G.; Borgonovi, G.E.; Jones, S.H.; Alexander, M. Anomalies in mineralization of low concentrations of organic compounds in lake water and sewage. Appl. Environ. Microbiol. 1986, 51, 226–232. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  5. Aksu, Z.; Kabasakal, E. Adsorption characteristics of 2,4-Dichlorophenoxyacetic acid (2,4-D) from aqueous solution on powdered activated carbon. J. Environ. Sci. Health B 2005, 40, 545–570. [Google Scholar] [CrossRef]
  6. Trillas, M.; Peral, J.; Domènech, X. Redox photodecomposition of 2,4-Dichlorophenoxyacetic acid over TiO2. Appl. Catal. B Environ. 1995, 5, 377–387. [Google Scholar] [CrossRef]
  7. Brillas, E.; Calpe, J.C.; Casado, J. Mineralization of 2,4-D by advanced electrochemical oxidation processes. Water Res. 2000, 34, 2253–2262. [Google Scholar] [CrossRef]
  8. Zona, R.; Solar, S. Oxidation of 2,4-Dichlorophenoxyacetic acid by ionizing radiation: Decomposition, detoxification and mineralization. Radiat. Phys. Chem. 2003, 66, 137–143. [Google Scholar] [CrossRef]
  9. Peller, J.; Wiest, O.; Kamat, P.V. Synergy of combining sonolysis and photocatalysis in the decomposition and mineralization of chlorinated aromatic compounds. Environ. Sci. Technol. 2003, 37, 1926–1932. [Google Scholar] [CrossRef]
  10. Aziz, K.H.H.; Miessner, H.; Mahyar, A.; Mueller, S.; Moeller, D.; Mustafa, F.; Omer, K.M. Degradation of perfluorosurfactant in aqueous solution using non-thermal plasma generated by nano-second pulse corona discharge reactor. Arab. J. Chem. 2021, 14, 103366. [Google Scholar] [CrossRef]
  11. Aziz, K.H.H.; Miessner, H.; Mahyar, A.; Mueller, S.; Kalass, D.; Moeller, D.; Omer, K.M. Removal of dichloroacetic acid from aqueous solution using non-thermal plasma generated by dielectric barrier discharge and nano-pulse corona discharge. Sep. Purif. Technol. 2019, 216, 51–57. [Google Scholar] [CrossRef]
  12. Aggelopoulos, C.A. Recent advances of cold plasma technology for water and soil remediation: A critical review. Chem. Eng. J. 2022, 428, 131657. [Google Scholar] [CrossRef]
  13. Bradu, C.; Magureanu, M.; Parvulescu, V.I. Degradation of the chlorophenoxyacetic herbicide 2,4-D by plasma-ozonation system. J. Hazard. Mater. 2017, 336, 52–56. [Google Scholar] [CrossRef]
  14. Singh, R.K.; Philip, L.; Ramanujam, S. Removal of 2,4-dichlorophenoxyacetic acid in aqueous solution by pulsed corona discharge treatment: Effect of different water constituents, decomposition pathway and toxicity assay. Chemosphere 2017, 184, 207–214. [Google Scholar] [CrossRef]
  15. Aziz, K.H.H.; Miessner, H.; Mueller, S.; Mahyar, A.; Kalass, D.; Moeller, D.; Khorshid, I.; Rashid, M.A.M. Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and non-thermal plasma using a planar falling film reactor. J. Hazard. Mater. 2018, 343, 107–115. [Google Scholar] [CrossRef]
  16. Maehara, T.; Miyamot, I.; Kurokawa, K.; Hashimoto, Y.; Iwamae, A.; Kuramoto, M.; Yamashita, H.; Mukasa, S.; Toyota, H.; Nomura, S.; et al. Decomposition of methylene blue by RF plasma in water. Plasma Chem. Plasma Process. 2008, 28, 467–482. [Google Scholar] [CrossRef]
  17. Amano, T.; Mukasa, S.; Honjoya, N.; Okumura, H.; Maehara, T. Generation of radio frequency plasma in high-conductivity NaCl solution. Jpn. J. Appl. Phys. 2012, 51, 978–982. [Google Scholar] [CrossRef]
  18. Ji, L. Research on Characteristics of Radio Frequency Plasma Discharge Underwater and Its Application of Azo Dyes Decomposition; Soochow University: Suzhou, China, 2012. (In Chinese) [Google Scholar]
  19. Wang, L.; Wang, J.; Lin, J.; Zhang, S.; Liu, Y. Decomposition of metronidazole by radio frequency discharge in an aqueous solution. Plasma Proces. Polym. 2018, 15, 1700176. [Google Scholar] [CrossRef]
  20. Zona, R.; Solar, S.; Gehringer, P. Decomposition of 2,4-dichlorophenoxyacetic acid by ionizing radiation: Influence of oxygen concentration. Water Res. 2002, 36, 1369–1374. [Google Scholar] [CrossRef]
  21. Gao, Q.; Liu, Y.; Sun, B. Characteristics of gas-liquid diaphragm discharge and its application on decolorization of brilliant red B in aqueous solution. Plasma Sci. Technol. 2017, 19, 115404. [Google Scholar] [CrossRef]
  22. Harvey, A.E.; Smart, J.A.; Amis, E.S. Simultaneous spectrophotometric determination of iron (II) and total iron with 1,10-phenanthroline. Anal. Chem. 1953, 26, 1854–1856. [Google Scholar] [CrossRef]
  23. Marotta, E.; Ceriani, E.; Schiorlin, M.; Ceretta, C.; Paradisi, C. Comparison of the rates of phenol advanced oxidation in deionized and tap water within a dielectric barrier discharge reactor. Water Res. 2012, 46, 6239–6246. [Google Scholar] [CrossRef]
  24. Li, X.; Jenks, W.S. Isotope studies of photocatalysis: Dual mechanisms in the conversion of anisole to phenol. J. Am. Chem. Soc. 2000, 122, 11864–11870. [Google Scholar] [CrossRef]
  25. Zona, R.; Solar, S.; Sehested, K.; Holcman, J.; Mezyk, S.P. OH-radical induced oxidation of phenoxyacetic acid and 2,4-Dichlorophenoxyacetic acid. Primary radical steps and products. J. Phys. Chem. A 2002, 106, 6743–6749. [Google Scholar] [CrossRef]
  26. Pignatello, J.J.; Oliveros, E.; MacKay, A. Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Crit. Rev. Environ. Sci. Technol. 2006, 36, 1–84. [Google Scholar] [CrossRef]
  27. Wang, L.; Jiang, X.; Liu, Y. Degradation of bisphenol A and formation of hydrogen peroxide induced by glow discharge plasma in aqueous solutions. J. Hazard. Mater. 2008, 154, 1106–1114. [Google Scholar] [CrossRef] [PubMed]
  28. Baxendale, J.H.; Dixon, R.S.; Stott, D.A. Reactivity of hydrogen atoms with Fe3+, FeOH2+ and Cu2+ in aqueous solutions. Trans. Faraday Soc. 1968, 64, 2398–2401. [Google Scholar] [CrossRef]
  29. Assadi, A.A.; Bouzaza, A.; Soutrel, I.; Petit, P.; Medimagh, K.; Wolbert, D. A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: From pilot to industrial scale. Chem. Eng. Processing Process Intensif. 2017, 111, 1–6. [Google Scholar] [CrossRef]
  30. Buxton, G.V.; Greenstock, C.L.; Helman, W.P.; Ross, A.B. Critical-review of rate constants for reactions of hydrated electrons, hydrogen-atoms and hydroxyl radicals (•OH/•O) in aqueous-solution. J. Phys. Chem. Refer. Data 1988, 17, 513–886. [Google Scholar] [CrossRef] [Green Version]
  31. Quint, R.M.; Park, H.R.; Krajnik, P.; Solar, S.; Getoff, N.; Sehested, K. γ-radiolysis and pulse radiolysis of aqueous 4-chloroanisole. Radiat. Phys. Chem. 1996, 47, 835–845. [Google Scholar] [CrossRef]
  32. Albarrán, G.; Mendoza, E. Radiolytic oxidation and degradation of 2,4-dichlorophenol in aqueous solutions. Environ. Sci. Pollut. Res. 2019, 26, 17055–17065. [Google Scholar] [CrossRef]
Figure 1. Experimental apparatus for 2,4-D degradation.
Figure 1. Experimental apparatus for 2,4-D degradation.
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Figure 2. Decays of 2,4-D and TOC and formation of Cl during RFD treatment. ([2,4-D]0, 1.0 mM; input power, 200 W; pH0, 6.51).
Figure 2. Decays of 2,4-D and TOC and formation of Cl during RFD treatment. ([2,4-D]0, 1.0 mM; input power, 200 W; pH0, 6.51).
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Figure 3. Variations of pH in the presence and absence of 2,4-D during RFD treatment (input power, 200 W; [2,4-D]0, 1.0 mM; initial pH, 6.51).
Figure 3. Variations of pH in the presence and absence of 2,4-D during RFD treatment (input power, 200 W; [2,4-D]0, 1.0 mM; initial pH, 6.51).
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Figure 4. 2,4-D removal under different initial pH values in buffered and unbuffered solutions (input power, 200 W; [2,4-D]0, 1.0 mM; discharge time, 10 min).
Figure 4. 2,4-D removal under different initial pH values in buffered and unbuffered solutions (input power, 200 W; [2,4-D]0, 1.0 mM; discharge time, 10 min).
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Scheme 1. H+ catalyzed cleavage of C1 ·OH adducts.
Scheme 1. H+ catalyzed cleavage of C1 ·OH adducts.
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Figure 5. H2O2 generation in the presence and absence of 2,4-D in RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 5.6).
Figure 5. H2O2 generation in the presence and absence of 2,4-D in RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 5.6).
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Figure 6. Effect of iron ions on 2,4-D removal in RFD treatment. (Conductivity 1.5 mS/cm, pH0 2.0, input power 200 W).
Figure 6. Effect of iron ions on 2,4-D removal in RFD treatment. (Conductivity 1.5 mS/cm, pH0 2.0, input power 200 W).
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Scheme 2. Fe3+ reduction by •OH-adducts formed by addition to the ring at positions unoccupied by the substituted groups (hydroxycyclohexadienyl radicals).
Scheme 2. Fe3+ reduction by •OH-adducts formed by addition to the ring at positions unoccupied by the substituted groups (hydroxycyclohexadienyl radicals).
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Figure 7. Formations of Fe2+ (a) and H2 (b) during RFD (input power, 200 W; pH0, 2.0).
Figure 7. Formations of Fe2+ (a) and H2 (b) during RFD (input power, 200 W; pH0, 2.0).
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Figure 8. Accelerating effect of RFD for Fenton-like process (input power, 200 W; pH0, 2.0).
Figure 8. Accelerating effect of RFD for Fenton-like process (input power, 200 W; pH0, 2.0).
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Figure 9. HPLC chromatograms during the RFD treatment (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
Figure 9. HPLC chromatograms during the RFD treatment (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
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Figure 10. Appearance and decay profile of 2,4-DCP during RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
Figure 10. Appearance and decay profile of 2,4-DCP during RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
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Figure 11. IC chromatogram of the RFD of 1.0 mM 2,4-D in pure water (input power, 200 W; discharge time, 40 min).
Figure 11. IC chromatogram of the RFD of 1.0 mM 2,4-D in pure water (input power, 200 W; discharge time, 40 min).
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Figure 12. Carboxylic acids determined by IC during RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
Figure 12. Carboxylic acids determined by IC during RFD (input power, 200 W; [2,4-D]0, 1.0 mM; pH0, 2.0).
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Scheme 3. Glycolic acid formation by cleavage of ipso-•OH-adducts on position 1 of 2,4-D.
Scheme 3. Glycolic acid formation by cleavage of ipso-•OH-adducts on position 1 of 2,4-D.
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Scheme 4. Formation of chloride ion (Cl) due to the addition of •OH on the position of 2 or 4, followed by the fast elimination of HCl.
Scheme 4. Formation of chloride ion (Cl) due to the addition of •OH on the position of 2 or 4, followed by the fast elimination of HCl.
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Scheme 5. Mechanism for formation of hydroxylated 2,4-Ds, 2,4-dichlorophenol and chlorohydroxyphenoxyacetic acid.
Scheme 5. Mechanism for formation of hydroxylated 2,4-Ds, 2,4-dichlorophenol and chlorohydroxyphenoxyacetic acid.
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Scheme 6. Mechanism for the formation of 4,6-DCR.
Scheme 6. Mechanism for the formation of 4,6-DCR.
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Scheme 7. Proposed decomposition pathway for 2,4-D decomposition in RFD.
Scheme 7. Proposed decomposition pathway for 2,4-D decomposition in RFD.
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Table 1. Energy yields for 2,4-D removal of RFD and other methods.
Table 1. Energy yields for 2,4-D removal of RFD and other methods.
C0 (mM)MethodkJ2,4-DReferences
min−1g/kWh
1.0RFD 200 W, pH0 2.00.0180.35This work
1.0RFD 200 W, pH0 2.0, 0.5 mM Fe3+0.0781.51This work
1.0RFD 200 W, pH0 2.0, 0.5 mM Fe2+0.0631.22This work
10.0RFD 200 W, pH0 2.0, 20 mM H2O2 2,4-D, 0.5 mM Fe3+0.0479.06This work
0.0045PCD over water surface, air as discharge gas (PCD/Air) 1.50[19]
0.25TiO2 photocatalysis, 125 W, 2.0g/L TiO2, pH0 3.3 0.76[6]
0.22Sonolysis, 50 W, O2 sparging 1.75[9]
0.45DBD/Ar-Fenton 8.83[15]
0.45DBD/Ar 3.85[15]
0.45DBD/Air, 200 W 0.25[15]
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Liu, Y.; Sun, B. Unusual Catalytic Effect of Fe3+ on 2,4-dichlorophenoxyacetic Acid Degradation by Radio Frequency Discharge in Aqueous Solution. Water 2022, 14, 1719. https://doi.org/10.3390/w14111719

AMA Style

Liu Y, Sun B. Unusual Catalytic Effect of Fe3+ on 2,4-dichlorophenoxyacetic Acid Degradation by Radio Frequency Discharge in Aqueous Solution. Water. 2022; 14(11):1719. https://doi.org/10.3390/w14111719

Chicago/Turabian Style

Liu, Yongjun, and Bing Sun. 2022. "Unusual Catalytic Effect of Fe3+ on 2,4-dichlorophenoxyacetic Acid Degradation by Radio Frequency Discharge in Aqueous Solution" Water 14, no. 11: 1719. https://doi.org/10.3390/w14111719

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